1. Field of the Invention
The present invention relates to a technology for embedding data into an image and extracting embedded data from a printed image.
2. Description of the Related Art
Recently, an electronic watermark technique of embedding invisible data into image data has been actively developed. For example, Japanese Patent Application Laid-Open No. 2004-349879 discloses a technique of dividing image data into plural blocks, and embedding plural codes into the blocks by relating one code to each block based on a magnitude relation of characteristics of the block such as average concentration of the block.
Japanese Patent Application Laid-Open No. H11-187245 discloses a technique of scanning a total image using a filter of a constant size, embedding data at plural positions where a predetermined condition is satisfied, and extracting data by scanning the image using the same filter, thereby detecting the positions where the predetermined condition is satisfied, and extracting the data.
However, according to the conventional techniques, at a time of reading a printed image to extract data, the data cannot be extracted correctly due to a distortion of the image, such as an enlargement or reduction or a rotation of the image, at a time of reading the image.
FIG. 19 is a schematic for illustrating a problem occurring when an image is reduced and rotated. In FIG. 19, y denotes a pixel of an area in which data is embedded, and x denotes a pixel of another area. When the image is reduced and rotated, the area embedded with the data does not coincide with a filtered area at the time of extracting the data. Therefore, the data cannot be correctly extracted.